1. Field of the Invention
This invention relates to digital audio processing.
2. Description of the Prior Art
Audible watermarking methods are used to protect an audio signal by combining it with another (watermark) signal for transmission or storage purposes, in such a way that the original signal is sufficiently clear to be identified and/or evaluated, but is not commercially usable in its watermarked form. To be worthwhile, the watermarking process should be secure against unauthorised attempts to remove the watermark.
The watermark signal may be selected so that it carries useful information (such as copyright, advertising or other identification data). It is a desirable feature of watermarking systems that the original signal can be restored fully from the watermarked signal without reference to the original source material, given the provision of suitable software and a decryption key.
EP-A-1 189 372 (Matsushita) discloses many techniques for protecting audio signals from misuse. In one technique, audio is compressed and encrypted before distribution to a user. The user needs a decryption key to access the audio. The key may be purchased by the user to access the audio. The audio cannot be sampled by a user until they have purchased the key. Other techniques embed an audible watermark in an audio signal to protect it. In one technique, an audio signal is combined with an audible watermark signal according to a predetermined rule. The watermark degrades the audio signal. The combination is compressed for transmission to a player. The player can decompress and reproduce the degraded audio signal allowing a user to determine whether they wish to buy a “key” which allows them to remove the watermark. The watermark is removed by adding to the decompressed degraded audio signal an equal and opposite audible signal. The watermark may be any signal which degrades the audio. The watermark may be noise. The watermark may be an announcement such as “This music is for sample playback”.
With a frequency-encoded (also referred to as “spectrally-encoded”) audio signal, for example a data-compressed signal such as an MP3 (MPEG-1 Layer III) signal, an ATRAC™ signal, a Phillips™ DCC™ signal or a Dolby™ AC-3™ Signal, the audio information is represented as a series of frequency bands. So-called psychoacoustical techniques are used to reduce the number of such bands which must be encoded in order to represent the audio signal.
The audible watermarking techniques described above do not apply to frequency-encoded audio signals. To apply—or to subsequently remove—an audible watermark, it is necessary to decode the frequency-encoded audio signal back to a reproducible form. However, each time the audio signal is encoded and decoded in a lossy system, it can suffer degradation.